A rotary kiln is a refractory-lined cylindrical vessel commonly used, for example, in the incineration of waste, in the calcining of cement, coke or other materials, in the firing of ceramic, and in many other uses. In the incineration of waste, the waste is provided into the kiln and is combusted while passing through the kiln by the combustion fuel and oxidant which is injected into the rotary kiln at one end of the kiln. The injection of the fuel and oxidant into the kiln may be either concurrent with the flow of waste or other material through the kiln, or it may be countercurrent to the flow of waste or other material through the kiln. Gases from within the kiln are removed through a flue located at one end of the kiln. After the waste has passed through the kiln, ash from the combusted waste is removed from the kiln.
In a countercurrent kiln the hot combustion gases and excess air are carried through the kiln first volatizing combustibles from the waste. These combustibles are combusted generating additional heat flowing countercurrently to the flowing waste which further dries the waste. It is imperative that the furnace gases contain sufficient mass to absorb the heat release without overheating which can cause refractory damage to the kiln or kinetically favor the generation of nitrogen oxides (NO.sub.x). Accordingly the throughput of material, such as waste, through the kiln is limited by the quantity of furnace gases generated within the kiln by the injected fuel and oxidant, and by the combusting volatiles if volatiles are present, and also by the rate at which heat can be transferred to wet material or to other heat sinks by the furnace gases.
In a concurrent kiln another problem arises in that the heat released from volatile combustibles is passing away from the wet material heat sink. An auxiliary burner is generally required to provide extra heat to the drying zone to dry the material so as to enable volatization of the volatile combustibles. This increases the volumetric flowrate of the gases passing out the flue increasing particulate carryover and burden on the air pollution devices thus limiting the throughput through the kiln.
The mismatch of heat source and heat sink which creates throughput limitations for both countercurrent and concurrent rotary kilns is more severe for long rotary kilns, such as kilns having a length to diameter (L/D) ratio exceeding 4.
A recent use for rotary kilns which has been gaining wide acceptance has been in the incineration of hazardous waste. A particularly advantageous rotary kiln for this application is a mobile or transportable rotary kiln which can be transported to the hazardous waste site and then removed when the hazardous waste site has been cleaned. Unfortunately a mobile rotary kiln is by necessity smaller than a stationary rotary kiln in order to enable transportability. Thus the throughput limitations discussed above are even more acute in the case of a mobile rotary kiln.
Accordingly it is an object of this invention to provide a rotary kiln having increased throughput over conventional rotary kilns without causing high potential for refractory damage or creating conditions highly favorable for NO.sub.x formation.
It is another object of this invention to provide a method for operating a rotary kiln so as to increase throughput over that obtainable with conventional rotary kiln operating methods without causing high potential for refractory damage or creating conditions highly favorable for NO.sub.x formation.